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Fair Value Measurements - Securities in Unrealized Loss Positions (Details) $ in Thousands | Dec. 31, 2020 USD ($) security | Sep. 30, 2020 USD ($) security
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Debt Securities, Available-for-sale [Line Items] |  |
Number of debt securities with unrealized loss | security | 33 | 51
Available-for-sale securities |  |
Less than 12 months, unrealized gross losses | $ (1,745) | $ (5,833)
12 months or more, unrealized gross losses | (2,108) | (3,781)
Total unrealized gross losses | (3,853) | (9,614)
Less than 12 months, fair value | 317,909 | 675,518
12 months or more, fair value | 195,084 | 285,161
Total, fair value | 512,993 | 960,679
Held-to-maturity securities |  |
Less than 12 months, unrealized gross losses | (4) | (52)
12 months or more, unrealized gross losses | 0 | 0
Total unrealized gross losses | (4) | (52)
Less than 12 months, fair value | 5,553 | 6,853
12 months or more, fair value | 0 | 0
Total, fair value | 5,553 | 6,853
Less than 12 months, unrealized gross losses | (1,749) | (5,885)
12 months or more, unrealized gross losses | (2,108) | (3,781)
Total unrealized gross losses | (3,857) | (9,666)
Less than 12 months, fair value | 323,462 | 682,371
12 months or more, fair value | 195,084 | 285,161
Total fair value | 518,546 | 967,532
Corporate debt securities |  |
Available-for-sale securities |  |
Less than 12 months, unrealized gross losses | (173) | (74)
12 months or more, unrealized gross losses | 0 | (405)
Total unrealized gross losses | (173) | (479)
Less than 12 months, fair value | 45,811 | 45,875
12 months or more, fair value | 0 | 24,596
Total, fair value | 45,811 | 70,471
Asset-backed securities |  |
Available-for-sale securities |  |
Less than 12 months, unrealized gross losses | (398) | (5,481)
12 months or more, unrealized gross losses | (1,699) | (3,027)
Total unrealized gross losses | (2,097) | (8,508)
Less than 12 months, fair value | 219,539 | 587,746
12 months or more, fair value | 141,478 | 204,369
Total, fair value | 361,017 | 792,115
Mortgage-backed securities |  |
Available-for-sale securities |  |
Less than 12 months, unrealized gross losses | (1,174) | (278)
12 months or more, unrealized gross losses | (409) | (349)
Total unrealized gross losses | (1,583) | (627)
Less than 12 months, fair value | 52,559 | 41,897
12 months or more, fair value | 53,606 | 56,196
Total, fair value | $ 106,165 | $ 98,093

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